Fix overwriting value of content's encoding attribute by type attribute.

Fix overwriting value of content's encoding attribute by type attribute.

[Issue#] N/A
[Problem] Though content encoding is specified the actual encoding value
is taken from type attribute if type attribute is set after encoding
attribute in the configuration file.
[Cause] Value from the last attribute parsed overwrites encoding of the
page even if it has been already read from encoding attribute.
[Solution] Check while parsing type attribute whether encoding has
already been set.
[SCMRequest] N/A

[Verification]
1. Build repository.
2. Run `wrt-extra-tests-w3c --output=text --regexp='ta_aaaaaaaaaa_z1'`

Change-Id: Ie49f4caf4111e3de00394a203acc3cee805d33de